DI3 interneurons maintain hindlimb motor tone in mice following spinal cord injury

Abstract

The dI3 population of spinal cord interneurons (INs) is a group of excitatory INs that receive varied sensory afferent input and project to ipsilateral motoneurons. Prior experiments have discovered that the permanent silencing of dI3 INs has a minimal impact on locomotor function in intact animals, but substantially interferes with locomotor rehabilitation/recovery after spinal cord injury (SCI). To gain insight into how dI3 INs are involved with locomotor function and recovery, the inhibitory DREADD receptor (hM4Di) was expressed in dI3 INs using a hybrid line of transgenic mice (Isl1-Cre:Vglut2-Flp x FlexloxFlexFRT-hM4Di). Consistent with prior experiments, transient inhibition of hM4Di-expressing dI3 neurons with the DREADD agonist JHU37160 (0.5mg/kg) did not significantly impact locomotor function in intact animals. However, in T9-T10 SCI mice, transient silencing of dI3 INs resulted in a significant loss of flexor motor tone, demonstrated by a significant increase in the resting ankle joint angle (+43.9° ± 11.6°, n=7, t-test, p=0.0065). This coincided with decreased stepping and the qualitative appearance of hindlimb flaccidity during the treadmill locomotor task. Based on these results, dI3 INs appear to adopt a more significant role in hindlimb function after SCI, specifically the maintenance of basal motor tone in the absence of supraspinal input. Given that SCI patients often experience both hypotonia and spasticity (increased motor tone), these findings provide the impetus for targeting dI3 INs to modulate motor tone below the level of the lesion.